1. Field of the Invention
The invention relates to a membrane electrode assembly for a tube-shaped fuel cell, which is used in a tube-shaped fuel cell, and a tube-shaped fuel cell, which can reduce costs and be made compact by being formed in a tube shape.
2. Description of the Related Art
A unit cell, which is the smallest power generating unit of a proton-exchange membrane fuel cell (PEMFC), typically has a membrane electrode assembly in which a catalyst electrode layer is joined to both sides of a solid electrolyte membrane. A gas diffusion layer is arranged on both sides of this membrane electrode assembly. Further, a separator having gas flow paths is arranged on the outside of the gas diffusion layer. Fuel gas and oxidant gas supplied the catalyst electrode layer of the membrane electrode assembly are then passed through the gas diffusion layers, and the electric current obtained by the power generated is then transmitted outside the cell.
The separator, which is sandwiched between the single cells, serves to prevent the fuel gas (e.g., hydrogen) entering the fuel electrode from mixing with the oxidant gas (e.g., air) entering the air electrode when cells are stacked together. The separator also acts as an electron conductor for connecting two cells together in series. A fuel cell which is used to generated power is made by stacking together the necessary number of single cells to assemble a fuel cell stack, and then integrating an apparatus to supply the fuel and oxidant gas, and a control apparatus, and the like.
With the structure of the proton-exchange membrane fuel cell (PEMFC) of the related art described above, however, a separator, i.e., a member used to both prevent the reaction gases between individual single cells from mixing and electrically connect the single cells together, is necessary. Because the solid polymer electrolyte membrane exhibits acidity due to the property of a sulfonic group, which is an ion-exchange group thereof, the separator must be both acid resistant and conductive. Therefore, carbon material or titanium material is used. These materials are both difficult to machine and costly, which increases the cost of the fuel cell main unit.
Also, in a flat fuel cell structure such as that described above, even if it were suited for a design in which a number of electrodes (i.e., fuel electrodes and air electrodes) with a large area are stacked up, the fuel cell could not be made compact. Thus, problems may occur in a case in which compactness is required, such as with a fuel cell to be mounted in a vehicle, for example.
In order to solve this kind of problem, a tube-shaped fuel cell has been proposed (see JP(A) 2002-289220, JP(A) 2002-124273, and JP(A) 2002-260685). This type of tube-shaped fuel cell does not require a separator and is therefore advantageous in that costs can be reduced and the fuel cell can be made compact.
In this type of tube-shaped fuel cell, however, it is necessary to have electrons that are generated flow in the axial direction of the tube. Unless a collector which is capable of making the electrons flow smoothly in the axial direction is provided, the efficiency will decrease.
Also, when attempting to form a collector using a tube-shaped conductor when attempting to form a membrane electrode assembly for a tube-shaped fuel cell using a solid electrolyte membrane which has been formed in a tube shape in advance, the dimensions for the outside diameter of the inside collector and the inside diameter of the outside collector must be extremely accurate. Further, in this case, the processes of mounting the inside collector and the outside collector are extremely troublesome and complicated.
In view of the foregoing problems, it is an object of the invention to provide a tube-shaped fuel cell and a membrane electrode assembly for a tube-shaped fuel cell that has a collector which has good power collecting efficiency in the axial direction of the tube, does not require high dimensional accuracy, and is relatively easy to mount.